1. Field of the Invention
The present invention relates to a rotary hydraulic device which utilizes a what is called balanced-type vane pump. More particularly, the invention relates to a rotary hydraulic device which is suitable for the power steerings of vehicles since the reduction of power consumption can be attained by restricting the increase of flow rate with the rise in the speed of rotation.
2. Description of the Prior Art
The delivery rate of a pump is varied in proportion to the speed of rotation thereof. In the pump which is mounted on a vehicle and is driven by the engine of the vehicle, the capacity of the pump must be made sufficient to supply necessary hydraulic fluid to the hydraulic machine such as a power steering device of the vehicle even when the speed of rotation of the engine is low, that is, when the rate of delivery of the pump is small. However, if the capacity of the pump is set to such the level, the hydraulic machine is supplied with excess fluid during the high speed rotation of the engine, which is wasteful.
In the conventional art, therefore, the delivery side of the pump is provided with a flow control valve and when the delivery of the pump exceeds a certain level, the excess fluid is returned to the tank, thereby maintaining the supply rate to the hydraulic machine at a constant level. In other pump device, after the supply rate is maintained at a certain level, the flow rate is further reduced. (For example, U.S. Pat. Nos. 3,426,785 and 3,314,495).
Nevertheless, since these flow control valves are actuated in response to the variation of the rate of delivery regardless of the pressure of the fluid delivered from the pump, the delivery quantity is large, and when the hydraulic machine is worked under the condition in which the fluid quantity is maintained below a certain level and the fluid pressure on the pump side is raised, the raised pressure works on all the pressurized fluid delivered from the pump. As the result, the pump must deliver a large quantity of high pressure fluid, which increases the power consumption of the engine. Further, the structure of the control valve of the above pump device in which the flow rate can be reduced, is complicated and the mechanical noises and pulsation are large, is addition, the flow rate to the hydraulic machine is liable to be varied by the fluctuation of pressure.